The rearrangement catalyzed by coenzyme-B12 dependent enzyme, alpha MG mutase, and an isomerase was studied with 2'-14C, 2'-3 H-alpha-methylene-glutaric acid. A partially purified enzyme preparation was found to convert alpha-methyleneglutarate to dimethylmaleate with no significant loss of tritium. A stereospecific synthesis of 2(R,S),5(S,R)-diaminohexanoic acid was achieved. This isomer was found to be the inactive isomer for the enzyme D-alpha-lysine mutase. Therefore, the enzymically active isomer must be 2R,5R-diaminohexanoic acid.